Endpointing detection for chemical mechanical polishing based on spectrometry
Methods and apparatus for spectrum-based endpointing. An endpointing method includes selecting a reference spectrum. The reference spectrum is a spectrum of white light reflected from a film of interest on a first substrate and has a thickness greater than a target thickness. The reference spectrum is empirically selected for particular spectrum-based endpoint determination logic so that the target thickness is achieved when endpoint is called by applying the particular spectrum-based endpoint logic. The method includes obtaining a current spectrum. The current spectrum is a spectrum of white light reflected from a film of interest on a second substrate when the film of interest is being subjected to a polishing step and has a current thickness that is greater than the target thickness. The method includes determining, for the second substrate, when an endpoint of the polishing step has been achieved. The determining is based on the reference and current spectra.
1. A computer implemented polishing method comprising:
storing at least one normalized predetermined spectrum;
polishing a substrate;
measuring a sequence of current spectra from the substrate undergoing polishing with an in-situ optical monitoring system;
normalizing each current spectrum in the sequence to generate a sequence of normalized current spectra, wherein normalizing includes determining a highest value and a lowest value within a range in the current spectrum and causing each normalized current spectrum of the sequence of normalized current spectra to have a peak-to-trough value equal to a peak-to-trough amplitude of the at least one normalized predetermined spectrum; and
determining a polishing endpoint using the at least one normalized predetermined spectrum and the sequence of normalized current spectra.
2. The method of claim 1 , wherein normalizing includes dividing by a spectrum of an underlying layer below an outermost layer being polished.
3. The method of claim 1 , wherein normalizing includes subtracting a spectrum representing the substrate being absent from over a sensor of the in-situ optical monitoring system.
4. The method of claim 1 , wherein normalizing includes calculating a normalized spectrum S that satisfies
S= ( A −Dark)/( Si −Dark)
where A is a current spectrum, Dark is a spectrum obtained when no substrate is placed over the in-situ monitoring system, and Si is the spectrum obtained from a bare silicon substrate.
5. The method of claim 1 , wherein normalizing includes calculating a gain G that satisfies
G= 1/( R max− R min)
where Rmax is the highest value and Rmin is the lowest value within the range.
6. The method of claim 1 , wherein the range is only a portion of the current spectrum.
7. The method of claim 6 , comprising receiving a user selection of the portion.
8. A computer implemented polishing method comprising:
polishing a substrate;
measuring a sequence of current spectra from the substrate undergoing polishing with an in-situ optical monitoring system;
for each current spectrum in the sequence, determining based on the current spectrum whether the current spectrum corresponds to a region of the substrate by sorting the current spectrum into one of a plurality of groups based on the shape of the current spectrum; and
determining a polishing endpoint using the current spectra that were not determined to correspond to the region.
9. The method of claim 8 , wherein the region is a scribe line.
10. The method of claim 8 , wherein the region is a non-array region.
11. The method of claim 8 , wherein a first group of the plurality of groups comprises current spectra from scribe lines of the substrate and a second group of the plurality of groups comprises current spectra from arrays of the substrate.
12. The method of claim 8 , further comprising averaging the current spectra that were not determined to correspond to the region.